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b=fbgXWj1/g519KQ+q0WyFpS2YospmjaHK7RkACo3duGiVboKd5uKktUCQnqfZvIIwG Bwn7XY1iAhz+VlC7yTOPEnHj1UQWBcbAUkNh+xXoknzNybHRuYZGKI7iOeOSkt+XmM REnH+JGDibRQzXQ5YgW0A5NMGkCWu1AGlPMOAjWFuZON8rsevFq5+B0l6t68Im0N/t bpYuIdDj2ztiVNHybuxqBqJ2cKoffhq3Lz1JDCprwnDqCJaHb2NrFljCC9FEgTYzmy OxoHRydNb6JFL8I8JL3+Tmk2ntpENq6Tr3/QsK5027s/x7n1JqPg0sZxRvTf4T0wSf i7GuITIgITgpw== Message-ID: <916d4173-36a3-48f2-8362-2ce7d937bee4@kernel.org> Date: Tue, 27 Oct 2026 13:15:35 +0100 X-Archived-By: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/ksm: fix userfaultfd protection during merge and unmerge To: Lance Yang , akpm@linux-foundation.org Cc: xu.xin@linux.dev, chengming.zhou@linux.dev, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, jglisse@redhat.com, aarcange@redhat.com, peterx@redhat.com, kas@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, bobbypowers@gmail.com, bgeffon@google.com, dplotnikov@virtuozzo.com, dgilbert@redhat.com, hughd@google.com, hannes@cmpxchg.org, kirill@shutemov.name, cracauer@cons.org, mcfadden8@llnl.gov, gokhale2@llnl.gov, mgorman@suse.de, mike.kravetz@oracle.com, rppt@linux.vnet.ibm.com, xemul@openvz.org, riel@redhat.com, shli@fb.com, jthoughton@google.com, corbet@lwn.net, pbonzini@redhat.com, seanjc@google.com, sj@kernel.org, ziy@nvidia.com, stable@vger.kernel.org, samarth.colleges@gmail.com, med08elkadiri@gmail.com References: <20261026130054.91461-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 45D158000A X-Rspam-User: X-Stat-Signature: 8n5iihrq4i437b6jajurobxwacjp9akt X-Spam-Status: No, score=-12.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,MAILING_LIST_MULTI,SPF_HELO_PASS,SPF_PASS, USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam03 X-HE-Tag: 1793103349-373294 X-HE-Meta: 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 4frjYAZP 7yUalMkz9+FLq9JtOvM4cy1QmPFTwFXv94rlAvLnNudVFmP5D0udJJfWAHZLoj8051YS9FGziG7pR/VW606xdAx8ynGyL2rp5PQv4EVDag/Gm0MuRCMNaDwRYwBzBeFXezfHgVD5TDVZqAdsJdbe5Bid/O899g82xhMR8a+Pz7mMVWjZcmOCIy3rJTPZaBvBxgPGKtThJHZVerQh3Qm+WkL5EgccvfjeIwRfuDCjzTRzBBKD7sCTljzCV6XvpvH8iLikF7/F++BpFW2I/t2U2wYtXkU74AjC05EpqBiT3WxyZ6KRBby7N1OEgebIr3OD6D2W4fyNnZGxT9k1MoVy3mZntty2BJypMuH9B0EeT0kOtKEgAm4/OMBtGxZqnFhDZePhm Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On 10/26/26 14:00, Lance Yang wrote: > KSM loses userfaultfd protection when replace_page() builds a new PTE > from vma->vm_page_prot. Dropping its uffd bit lets a subsequent write go > through ordinary COW instead of userfaultfd write protection handling. For > RWP, a merged page also becomes readable without faulting. > > Keeping the uffd bit and restoring PAGE_NONE fixes merging, but we also > have to handle unmerging. An existing KSM page can be protected with RWP > before MADV_UNMERGEABLE, so unmerging already needs fixing independently > of preserving RWP during merging. > > Unsharing replaces a shared page with a private one and should leave > userfaultfd protection in place, as do_wp_page() already does for write > protection. Currently, an unshare fault reaches do_uffd_rwp() before it > can proceed to do_wp_page(). > > - With synchronous RWP, handle_userfault() returns VM_FAULT_SIGBUS. > break_ksm() then reports success with a KSM page still merged. > > - With asynchronous RWP, protection is cleared before unsharing. > > Preserve the uffd bit when merging, and restore PAGE_NONE for RWP. Let > FAULT_FLAG_UNSHARE skip RWP fault handling and keep PAGE_NONE on each > private copy as well. Unsharing can then complete with RWP still armed > for a subsequent access. > > Fixes: 63b2d4174c4a ("userfaultfd: wp: add the writeprotect API to userfaultfd ioctl") > Fixes: 2d427bb01639 ("mm/userfaultfd: add RWP fault delivery and expose UFFDIO_REGISTER_MODE_RWP") > Link: https://lore.kernel.org/all/20261019010934.697895-1-samarth.colleges@gmail.com/ > Link: https://lore.kernel.org/all/20261020055518.36087-1-lance.yang@linux.dev/ > Cc: stable@vger.kernel.org > Co-developed-by: Samarth Narang > Signed-off-by: Samarth Narang > Signed-off-by: Lance Yang Acked-by: David Hildenbrand (Arm) -- Cheers, David